Conventionally, in order to encipher and transmit image data, the image data as a whole is encrypted or scrambled. This technique involves pre-encrypting the image data as a whole using a single encryption key, so that the data can only be correctly decrypted by a decryption key corresponding to the encryption key.
However, with image data having a layered structure, it is desirable to encrypt the image data using a different encryption key for each layer of that layered structure instead of encrypting the image data as a whole, in order to control the ability to reproduce image data that corresponds to that layered structure. Moreover, since the image data is composed of a plurality of tiles, it is desirable to encrypt the data using a different encryption key for each tile, in order to control the ability to reproduce each tile. Furthermore, where the image data is composed of a plurality of tiles and each individual tile has a layered structure, a different encryption key is used to encrypt each layer in each tile in order to control the ability to reproduce the image data that corresponds to the tiles and the layered structure.
By thus encrypting the image data using a different encryption key for each tile and each layer as described above, it is possible to control the reproduction of the image data with each tile and each layer.
However, where the encoded tile data is composed of a plurality of partially encoded pieces of data, and each individual partially encoded piece of data is encrypted using a different encryption key, the layered structure of the partially encoded pieces of data and the decryption key for its decryption must be the same, which inevitably complicates key information management.
Moreover, it is obvious that the image data cannot be properly decrypted if the keys for each partially encoded pieces of data are not properly managed.